Zenith Console combo - Page 4 - Videokarma.org TV - Video

ctc17 · #46 08-19-2010, 11:06 PM

I took a look and thats exactly what it looks like to me, low frequency coupling. I would leave it and listen to some music programing and see how it sounds.

4 IF stages, Nice! I never really looked at the schematic because both of mine work perfect.

DaveWM · #47 08-20-2010, 07:07 PM

still not satisfied with the lack of separation, this time over the air MPX fm, I got out my little rat shack stereo transmitter, hooked up a dual trace scope thru the tape out and applied a 400hz tone one channel at a time to the transmitter.

ugh, about 2 to 1 amplitude on the L and R.

I tweeked the 38kc coil, got to about 8 to 1 (best I could get) checked the other channel same ratio. Maybe some drift in a resistor, but only turned it less than a 1/4.

I may check all the transistors in the mpx circuit for gain, just for fun but I think is pretty darn good now for separation. for fun I may put the resistor that ties the two bases on the preamp back in just to see how much it effects the separation when using the transmitter.

DaveWM · #48 08-20-2010, 07:14 PM

ah much better. I connected that resistor back in the circuit and the scope showed BETTER separaion. I dont know whats going on, but it sounds MUCH better now with some actual over the air listening. Before you had to really listen to hear the separation, now its fine like you would expect.

There is a bit of a volume difference, this shows up on the scope as one channel being slightl higher amplitude. It can be compesated of course with the balance, but I would rather see if I can find the problem. there are a couple 5% resistors that feed the MPX detector that look like a likely suspect.

I should be interesting to see how the other tuner stacks up against this one.

DaveWM · #49 08-20-2010, 07:45 PM

ok rookie mistake #3 make sure on a dual trace that the voltage adj pot has been set. after setting up the trim pots for an equal vert on the same voltage the curves lined up perfect. this is at the tape out which is after the 1st stage of AF amp but before the volume control. I suspect my volume control is just not tracking. so next step will be to scope after the volume control and see how the curves line up.

DaveWM · #50 08-20-2010, 07:54 PM

argh just too hard to get a clean signal, too much ac hum. heck with it, if it is the loudness there is nothing I can do about it anyway. There are a couple emitter bypass caps 100uf, maybe something there, if one is weak it could cause neg feedback.

opps already did that. ok DONE...

time to get on the TV....

DaveWM · #51 08-21-2010, 08:22 AM

pic of the separation, this is with a tone on one channel of the FM stereo transmitter, scopt to the tape out.

sounds GREAT now!

DaveWM · #52 08-21-2010, 09:47 AM

come to think of it I have a fisher 400 that I have never been to keen on the separation. the tape output/dual scope/stereo xmitter/tone generator is a pretty good way to set that 38khz coil. When I get some of these TV's out of the way I may have to go back and work on that one.

ctc17 · #53 08-21-2010, 01:27 PM

I really think removing that resistor is a mistake. I checked both my combos again last night and they have great stereo separation with both channels going. With only one, yes there is some bleed over but it seems to go competently away with both signals.

The fm multiplex we use is the Zenith system and invented by Zenith. They knew how to do Stereo and I really doubt they would intentionally cripple their unit with a single resistor.

DaveWM · #54 08-21-2010, 01:38 PM

forgot to mention, I did put it back, and it did IMPROVE the stereo separation with the MPX. Not sure what the deal was but yes you are correct, leave it as designed. that last scope pic was with the resistor back in.

ctc17 · #55 08-21-2010, 02:32 PM

It appears they have some fancy phasing going on there where it blends the channels if one is missing and increases separation if both are there. I will study the circuit when I get time.

Stereo is one of those arts that has faded and been killed in modern music and programming. They got a 60/40% maximum separation rule they go by now. We wouldnt want all those home wired car stereo installs where all the speakers are out of phase and hooked up to the left channel to miss part of the crappy song.

I would like to implement something like this resistor deal at the dance club I am the sound tech for. That way when the djs crappy equipment drops a channel it blends and doesn't kill the party yet its still stereo when the information is there. I have gone to lengths to keep that place stereo, you really cant find a canned music venue anywhere that's true stereo. In fact most of the new equipment out of the box is default mono.

DaveWM · #56 08-21-2010, 03:55 PM

prob why the best separtion can be found on old early 70's rock. I listen to the classic rock station and get a good dose of separation (think whole lotta love by led zep).

ctc17 · #57 08-21-2010, 06:09 PM

Beetles and Doors weren't too shabby. Phil Spector cuts things up real nice too.

DaveWM · #58 08-21-2010, 08:41 PM

ok next up the phono, went to power it up on the variac, instant 3 amps at 10v, opps thats not good. pull the phono out and the motor coil is all toasted melted etc. guess that is what blew the fuse. the motor armature was seized up. pulled the motor and tried the turntable by hand, spent a couple hours finding out the issue there (it would not cycle properly) one tine little arm was out of place and the spring goofed up. after lubing and check reinstalling the spring is cycled correctly. Good thing too, there was a zenith TT on ebay with a starting bid of 1.99, the shipping was high but what the heck, well next thing you know the dumb thing is over 40$ bid and one day left. Needless to say there is no way I will pop for 70$ plus (inc shipping) for a cheezy zenith TT. So now I have to get this one fixed. I am pretty sure I can get a replacement motor (one of those induction motors with the stepped shaft that drives and idler pulley) from my local vintage guy (Dennis at radio relics) or Sky junk suplus. the issue maybe how to get that stepped driver off the shaft. It looks to be pressed on, too bad it did not use a set screw.

anyhoo I know the cart works (spun a record by hand) so I will deal with the motor next week.

I prob should have started with the TV at this rate it seems like it has to be easier that this TT (all automatic TT's were designed by the devil himself IMHO).

I would like to have know the failure sequence when this thing went bad. As badly as the motor is toasted I am suprized the fuse would not have cut it off before it melted down (it still had the orig fuse in, blown but in).

radiotvnut · #59 08-21-2010, 09:06 PM

Try www.thevoiceofmusic.com. Most Zenith turntables from this era were built by VM and Gary can likely fix you up with a motor for a reasonable price. If not, any motor from a similar vintage VM record changer should work. And, I don't blame you for not plunking down $70 on a basic record changer. VM built changers were very common back then; and, you will find one much cheaper if you look.

radiotvnut · #60 08-21-2010, 09:08 PM

I just looked at the pictures again and that is a VM built changer. Try that site I mentioned above and I'll be on the lookout for you a motor, as well.